feat(stellar): add buildStellarSwapAndStealth helper

[Mathew2k-hash]

feat(stellar): atomic swap + stealth payment helper

Adds buildStellarSwapAndStealth — a single-envelope Stellar transaction that swaps one asset
for another via the AMM and delivers the output to a one-time stealth address, with a Wraith
announcement, all atomically.

What's in the transaction

• Native XLM as toAsset (2 ops): pathPaymentStrictReceive delivers XLM directly to the stealth
  address (creating the account if needed) + invokeHostFunction announcement.
• Non-native toAsset e.g. USDC (3 ops): pathPaymentStrictReceive → sender,
  createClaimableBalance claimable by the stealth address (no trustline required), +
  announcement.

Slippage protection

sendMax caps the maximum spend. If the AMM path costs more, Stellar rejects the transaction
before any funds move. The docs include a snippet for deriving sendMax from a Horizon
/paths/strict-receive quote with a configurable basis-point tolerance.

Files changed

• src/chains/stellar/swap.ts — implementation
• src/chains/stellar/index.ts — exports buildStellarSwapAndStealth,
  BuildStellarSwapAndStealthOptions, SwapAndStealthResult
• test/chains/stellar/swap.test.ts — 7 unit tests (native path, non-native path, slippage
  encoding, determinism, announcement always present)
• test/chains/stellar/swap.integration.test.ts — 3 testnet tests against live Horizon/AMM pools
  (skipped unless INTEGRATION=1)
• docs/guides/stellar-swap-and-stealth.mdx — full guide with API reference, worked examples,
  slippage calculation, and recipient claim flow

Testing

pnpm test test/chains/stellar/swap.test.ts # 7/7 passing
INTEGRATION=1 pnpm exec vitest run test/chains/stellar/swap.integration.test.ts

closes #85

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[truthixify]

Clean merge. buildStellarSwapAndStealth (DEX swap then send to stealth address atomically) is exactly the helper the demo's swap-and-pay flow needs. Tests cover the integration path. Thanks @Mathew2k-hash.